A network constituting a control system may be made to be redundant in order to achieve high reliability, and, as a redundancy method using a ring network, there are methods called High availability Seamless Redundancy (HSR) and Parallel Redundancy Protocol (PRP) in IEC 62439-3. As an example using the HSR method, NPL 1 discloses that a transmission device transmits identical frames from ports clockwise and counterclockwise in a ring network, and the identical frames are transmitted to a reception device via clockwise and counterclockwise paths of the ring. Consequently, even if a failure occurs in a part of the network, the reception device can receive the frame without any loss via a route not including the failure location.